The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Centrifugal pumps are used for many applications including pumping water through reverse osmosis systems. In FIG. 1, a single-stage centrifugal pump 10 is illustrated. The pump 10 includes a casing 12 that includes an inlet 14, impeller chamber 16, and an outlet 18. The casing also includes a bearing portion 20.
The pump 10 has a shaft 22 that is supported within the casing 12 by bearings 24. The bearings 24 provide radial location of shaft 22. The bearings are located within the bearing portion 20.
The shaft 22 is coupled to an impeller 26. As the shaft rotates, the impeller spins generating the pumping action. The shaft 22 is coupled to a motor 28 that is used to rotate the shaft 22. A coupling 30 is used to couple the motor 28 to the shaft 22.
The impeller 26 is coupled to the impeller end of the shaft 20 while the bearings are located at the motor end of the shaft 22. The impeller end may also be referred to as the outboard direction while the motor end of the shaft is referred to as the inboard direction.
Located radially outward from the shaft 22 and the impeller 26, a volute volume 32 is formed within the impeller chamber 16. The volute volume 32 surrounds the peripheral of the impeller 26. The impeller chamber 16 also includes an outboard impeller side chamber 34 and an inboard impeller side chamber 36.
The impeller 26 may also include an impeller wear ring 40 that extends axially from the impeller toward the inlet and is concentric with the shaft 22. The casing 12 may include a casing ring 42 disposed directly adjacent to the impeller wear ring. A close clearance passage with the impeller ring 40 is formed by the casing ring 42. Fluid flows into the device in the direction illustrated by arrow 44. Fluid flows out from the pump 10 through the outlet 18 and through a diffuser 46 in the direction of arrow 48. As the pump spins, a net force indicated by arrow 50 is provided.
A shaft seal 52 isolates the impeller chamber 16 from the bearing portion 20. Thus, fluid within the impeller chamber 16 does not enter the bearing portion 20.
The motor 28 causes the pump shaft 22 to rotate the vanes 56 of the impeller 26 rotate and engage the entrained fluid causing a tangential velocity for rotation of the fluid. The rotation of the fluid imparts a radial flow causing the fluid to flow into the impeller 26 through the inlet 14 in the direction of arrow 44. Fluid exits the impeller 26 with a combined radial and tangential velocity component. The volute volume 32 accepts and directs the flow to the diffuser 46. The diffuser 46 reduces the fluid velocity and recovers a portion of the dynamic pressure in the form of static pressure. The fluid exits the diffuser 46 through the outlet 18.
In addition to radial loads on the shaft created by the weight of the impeller and the shaft, a very large force can act on the shaft in the axial direction. The axial force may be derived from two sources. The first source is the high pressure at the inlet 14 that can push the impeller 26 and the shaft 22 toward motor 28. The second source of axial force is present during the rotation of the impeller 26. The rotation of the impeller may generate a pressure at the outboard impeller side chamber 34 and the inboard impeller side chamber 36. Typically, less pressure is developed at the outboard impeller side chamber when compared to the inboard impeller side chamber due to the wear ring 40. A pressure inboard on the impeller 26 may result in the net force illustrated by arrow 50 in the outward or outboard direction. The axial force induced by the impeller rotation is typically much greater than the force generated by the pressure into the inlet 14 illustrated by arrow 44, thus a net axial force indicated by arrow 50 may result.
The bearing 24 may be various types of bearings including a roller contact-type bearing, such as ball bearings using oil or grease lubrication. When bearings 24 using oil or grease lubrication are present, a shaft seal 52 isolates the pressurized fluid in the impeller chamber 16 from the bearing 24. The bearings 24 also accommodate both axial thrust and radial thrust forces.